In an electron gun, the surface of the electron-emissive coating of a cathode is axially positioned relative to a control grid or G.sub.1. The cathode is affixed to a tubular eyelet and the eyelet and grid are fixed to and electrically insulated from one another by glass support rods. The electron gun includes also a screen grid or G.sub.2 which is spaced from the G.sub.1 on the side opposite the cathode.
When the emissive surface of the cathode is positioned too close to the control grid, arcing between the cathode and the control grid may occur, and the cutoff voltage may change. A very small error in the spacing distance, such as 0.001 inch (0.0254 mm) can change the cutoff voltage of the electron gun by about 60 volts.
There are several methods presently in use for establishing cathode-to-grid spacing. In the most common an air probe is inserted through the grid apertures and the cathode is moved toward the probe until a predetermined back pressure, related to spacing, is measured. Other methods have utilized optical, microscopic and capacitance measurements to set spacing. Other methods have used soluble or volatizable spacers which are removed after the desired spacing is achieved or permanent spacers which remain.
Cathode-grid spacings set by permanent spacers can vary from tube to tube because of part and assembly tolerances. Gas flow gages require the insertion of a nozzle through the grid aperture which can damage that aperture. Optical measurements are too slow for production. Spacer materials suggested by the art have been difficult to remove and no adequate method of attaching such a spacer to the potentially emissive material has been disclosed.
Capacitance measurements do not work well when the spacing tolerance is small because the capacitance change being sensed is between the grid and cathode metal and not the emissive surface.
U.S. Pat. No. 4,176,432 discloses a method whereby a non-deformable and a deformable sheet of material, whose total thickness equals the spacing desired, are employed. Cathodes are inserted until they penetrate the deformable strip, then the non-deformable strip is removed and then the deformable strip is removed. This method introduces two different materials whose thickness must be controlled and provides no consistent guidelines for the depth of penetration.